1. Field of the Invention
The present invention relates to a rotating-body driving device for driving a rotating body and an image forming apparatus, such as a copier, a printer, or a facsimile machine, equipped with the rotating-body driving device.
2. Description of the Related Art
In general, as a color image forming apparatus, there is known, for example, a direct transfer method of tandem-type image forming apparatus that forms a color image on a recording medium by transferring solid black (Bk), yellow (Y), magenta (M), and cyan (C) images formed on photosensitive drums onto the recording medium being carried/conveyed on a recording-medium conveyance belt in a superimposed manner.
In such a tandem-type image forming apparatus, generally, a motor provided as a driving source of a photosensitive drum and a reduction gear unit are installed in the main body side of the image forming apparatus, and the reduction gear unit is connected to the photosensitive drum so as to transmit a driving force to the photosensitive drum.
Furthermore, in the tandem-type image forming apparatus, the quality of an image is greatly affected by the accuracy of surface moving speed of each photosensitive drum. Respective transfer positions of solid color images on the photosensitive drums are relatively shifted by fluctuation in surface moving speed which periodically occurs in the individual photosensitive drums. This causes a color shift of a color image formed on a recording medium or a so-called “banding phenomenon”, density unevenness that periodically appears like strips, in a range of the color image formed.
Such fluctuation in surface moving speed is caused by an error in transmission of a drive transmission system installed on a shaft of a photosensitive drum (a transmission error due to gear eccentricity or cumulative tooth pitch deviation, and the like) and a transmission error due to a coupling provided to removably attach the photosensitive drum to the drive transmission system (axial tilt, shaft misalignment, and the like).
The periodic fluctuation in surface moving speed occurs with a rotation period of the shaft, a rotation period of gears, and a rotation period of a higher-order component, and constantly occurs in a driving state. Furthermore, the magnitude of the periodic fluctuation in surface moving speed varies according to the progression of gear wear with time or changes in installation conditions, such as a hygrothermal environment, of the image forming apparatus. Therefore, to correct the color shift, it is necessary to suppress the periodic fluctuation in surface moving speed of the photosensitive drums that varies with time and environment.
For example, Japanese Patent No. 2754582, Japanese Patent No. 3259440, and Japanese Patent Application Laid-open No. 2008-099490 have disclosed a technology of detecting an angular velocity of a shaft of a photosensitive drum when a drive motor is rotated at a predetermined constant angular velocity with a rotary encoder, storing information on fluctuation in angular velocity of the photosensitive drum during one revolution, and changing the angular velocity of the drive motor on the basis of the stored fluctuation information at the timing of a home position signal output with each rotation of the rotary encoder, i.e., executing so-called feedforward control. This technology can eliminate an oscillation phenomenon such as an increase in rotation speed fluctuation which is a concern in feedback control and achieve stable drive control, and therefore can suppress periodic fluctuation in surface moving speed of the photosensitive drum.
Furthermore, Japanese Patent Application Laid-open No. 2010-008924 has disclosed a technology of counting the number of pulses output from a rotary encoder and outputting a timing signal when it comes to a pulse count corresponding to one revolution of the rotary encoder, thereby detecting a home position signal.
In this manner, to perform feedforward control on the basis of fluctuation information detected in advance based on a preset home position on a rotating shaft is effective as a method to suppress periodic fluctuation in surface moving speed of a photosensitive drum.
However, in such a conventional image forming apparatus using feedforward control based on a home position in each rotation of a rotary encoder (in each rotation of a photosensitive drum), a period of fluctuation to be corrected is limited to only an integral period with respect to a rotation period of the photosensitive drum.
Therefore, if a reduction gear with a non-integral reduction gear ratio, for example, a planetary gear mechanism is adopted, there is a problem that there exist gears with a non-integral ratio or non-terminating decimal ratio of rotation period, and fluctuation cannot be corrected on the basis of the home position in each rotation of the photosensitive drum.
As a means for feedforward control of gears with a non-integral ratio or non-terminating decimal ratio of rotation period, for example, a home position could be set on each gear.
However, this configuration has a problem that a component for detecting a home position of each gear has to be installed, which results in an increase in the number of parts and an increase in cost.
There have been needs to solve these problems and to provide a rotating-body driving device capable of performing feedforward control enabling, even when a reduction gear having gears that each rotate with a non-integral ratio of rotation period to a rotation period of a shaft of a photosensitive drum is used in a drive transmission system of the photosensitive drum, to suppress periodic fluctuation generated with the respective rotation periods of the gears.